The present invention is directed in general toward token passing local area networks (LANs) and, more particularly, toward a method for employing a plurality of tokens in a LAN to increase the bandwidth utilization thereof.
Local area networks, wherein a plurality of bus interface units (BIUs) couple their respective users to a common transmission bus for communication therebetween, are becoming more and more prevalent in today's world. With this widespread acceptance comes the need to improve both the speed and efficiency of these systems. The present invention provides a method for increasing the bandwidth utilization of the common transmission bus in a token passing LAN.
Bandwidth utilization as used herein refers to the percentage of time that the transmission bus is used for the transfer of information, e.g. maximum bandwidth utilization would refer to constant transfer of information along the bus or 100% bus loading.
Communication between users in a LAN is accomplished via their respective BIUs using generally two protocols, to wit, a contention protocol and a token passing protocol. Generally, the contention protocol allows all BIUs having a message, or information to transmit to contend for use of the transmission bus. However, this method leads to uncertainty and delays where two or more BIUs contend for use of the bus simultaneously. Therefore, during intervals where many BIUs have messages to transmit, many conflicts occur decreasing the effective bandwidth of the transmission bus. Further, this method does not provide equal access to the bus or allow for prioritization of messages.
In the token passing scheme, a token packet is passed from BIU to BIU and each BIU is permitted to transmit information packets only while in control of the token. While this method provides equal access to the transmission bus, substantial bandwidth is wasted during intervals of high bus utilization. This is because the bus cannot be used, and therefore bandwidth is wasted, from the time a BIU receives the token until the time that BIU begins transmission of its information packet. Further, like the contention protocol, this method does not allow for prioritization of urgent information.